The present invention relates to a multi-pressure reheat combined cycle configuration and, in particular, such a combined cycle configuration in which cycle output and efficiency are improved.
The optimization of steam cycle conditions for a combined cycle (CC) steam plant is a strong function of the constraints placed on the evaluation. A key constraint is the configuration of the surfaces within the heat recovery steam generator (HRSG), which relate not only to the gross cycle configuration, that is one pressure vs. two pressure vs. three pressure cycle, or reheat vs. non-reheat, etc., but also finer scale details of achievable steam conditions and cost/performance trade-off studies.
Traditionally, studies of optimal reheat pressure for a three pressure reheat bottoming cycle have been performed with the reheat sections of the HRSG constrained to be upstream of the HP evaporator section with respect to the exhaust gas flow. An exemplary such HRSG is shown and described, for example, in U.S. Pat. No. 5,628,179, the disclosure of which is incorporated herein by this reference. These studies showed that cycle output peaked at approximately 20-25% P.sub.CRH /P.sub.THROTTLE. This result was obtained for a three pressure reheat cycle wherein the IP superheater discharge steam is combined with the cold reheat (CRH) steam from the steam turbine and sent to the reheater. A similar result was obtained as well with two and three pressure cycle variations wherein the IP steam was generated at a pressure less than P.sub.CRH and admitted to an IP turbine admission.